This is a sequel to our Anatomy of Switching Power Supplies tutorial and we are going to explore in depth all power supply protections like over voltage (OVP), under voltage (UVP), over current (OCP), over power (OPP), over load (OLP), over temperature (OTP), no-load operation (NLO) and also the power good signal.

Usually power supplies have a monitoring integrated circuit on their secondary (see Figure 1), which is in charge of the power supply protections. Protections can also be built using stand-alone components instead of using a ready-made integrated circuit – the most common integrated circuit for this option is the LM339, which is a voltage comparator. Frequently the monitoring circuit is built on a small printed circuit board that is attached to the main printed circuit board from the power supply.

click to enlargeFigure 1: Monitoring integrated circuit.

On power supplies based on the outdated half-bridge topology, protections can be provided by the PWM controller, which is physically present on the secondary. Some half-bridge power supplies with better design will provide a monitoring integrated circuit in addition to the PWM controller.